Self-bearing step motor and its control method

ABSTRACT

The present invention relates to a self-bearing step motor, and more particularly, to self-bearing step motor system and a control method thereof, wherein a rotor can be supported by magnetic force without adding an additional winding for supporting the rotor to a step motor having no mechanical bearing for supporting the rotor. The self-bearing step motor system of the present invention comprises a self-bearing step motor including a rotor having a plurality of rotor teeth, a stator having a plurality of stator teeth, a plurality of separate windings wound respectively around the plurality of stator teeth, and a plurality of sensors for measuring geometric deviation of the rotor; and a controller which transforms a torque current for causing the rotor to rotate and control currents for performing a bearing function of supporting the rotor into predetermined supply currents capable of generating magnetic force for allowing the rotor to be supported while causing the rotor to rotate in accordance with outputs of the sensors and position angles of the plurality of windings, which are fed back to the controller, and simultaneously distributes the supply currents among the plurality of windings grouped so that the same phase currents can be supplied thereto. According to the present invention, the self-bearing step motor can replace the conventional step motor. In particular, owing to advantages of the magnetic levitation technology, the self-bearing step motor can be used semipermanently even in a severe environment needed for a super clean and vacuum system without maintenance thereof. Thus, a wide industrial application thereof can be obtained.

TECHNICAL FIELD

The present invention relates to a self-bearing step motor, and moreparticularly, to a self-bearing step motor system and a control methodthereof, wherein a rotor can be supported by magnetic force withoutadding an additional winding for supporting the rotor to a step motorhaving no mechanical bearing for supporting the rotor.

BACKGROUND ART

In general, a self-bearing motor is called a bearingless motor or anintegrated motor bearing. Since the self-bearing motor does not need touse any mechanical bearings, its volume and weight can be reduced.Further, since the self-bearing motor utilizes a magnetic levitationtechnology, friction and wear thereof can be minimized. Furthermore,since there is no need for lubrication in the self-bearing motorcontrary to the mechanical bearings, effective maintenance andsemipermanent use thereof can be made.

FIG. 2 shows a sectional view of an asymmetric conventional 3-phase stepmotor including a stator 20 with fifteen stator teeth 21 and a rotor 10with ten rotor teeth 11. Coils (windings) 30 are wound around the statorteeth. As shown in FIG. 2, the conventional step motor is constructedsuch that the rotor 10 is rotated by magnetic force while coils, whichare connected in series and supplied with same phase currents, areexcited in alternating order by means of three phase currents suppliedfrom a step motor controller 40. When the coils are excited by therespective phase currents, in case of the step motor shown in FIG. 2,five electromagnets connected in series are simultaneously excited. Inthe asymmetric step motor, when the coils are excited by the respectivephase currents, the electromagnets for generating electromagnetic forceare not arranged symmetrically around the circumference of the statorcircle.

A self-bearing step motor, which is supported by magnetic force byadding an additional winding without any mechanical bearings, isdisclosed in detail in U.S. Pat. No. 4,683,391 entitled “magneticallyfloating actuator having angular positioning function” issued to ToshiroHiguchi. FIG. 1 is a schematic view of the self-bearing step motor ofthe '391 patent As shown in FIG. 1, a technique disclosed in the '391patent is characterized in that a single actuator was allowed to performthe motor and bearing functionalities simultaneously by adding bearingcoils (bearing windings) 60 for generating the magnetic force to supporta rotor 80 to torque coils (torque windings) 50 of electromagnets of theconventional step motor. Further, the '391 patent discloses that X-axisand Y-axis motions of the rotor 80 are controlled in a state where astator 70 is divided into four segments.

However, in the technique disclosed in the '391 patent, the structure ofthe stator 70 should be separately designed to accommodate the windingsof the bearing coils, and additional bearing coils and equipments forcontrolling the additional bearing coils are needed. Thus, thesubstantial effect of reduction in volume and weight of the step motorowing to elimination of the mechanical bearing is not too great. Inparticular, there is a problem in that the technique is applicable onlyto a stator having a symmetric structure.

A self-bearing step motor, which neither uses a mechanical bearing norincludes additional windings for supporting a stator, is disclosed inU.S. Pat. No. 5,424,595 entitled “integrated magnetic bearing/switchedreluctance machine” issued to Mark A. Preston et al. The self-bearingstep motor disclosed in the '595 patent is characterized in that itcomprises a rotor having rotor teeth and a stator having stator teethwound with windings and the windings are separated from one another andsimultaneously excitable. Further, the self-bearing step motor of the'595 patent is characterized in that the same phase currents aredistributed among the respective windings in an inversely proportionalmanner. Furthermore, it is characterized in that the stator teeth aredisposed in diametrically opposite pairs.

However, the '595 patent is merely directed to a hetero polar type ofstep motor. In such a case, it is difficult to perform stable control ofthe self-bearing step motor, because a magnetic flux direction ischanged upon rotation of the rotor. In particular, it is difficult toapply such a self-bearing step motor to a case where the stator teethare a symmetrically arranged.

The features of the conventional magnetic levitation technology are thatthe windings are symmetrically disposed to magnetically float an objectas shown in FIG. 1. Since the electromagnets are disposed to besymmetric with respect to the X- and Y-axes, four electromagnets aregenerally needed for performing a magnetic bearing function.

On the other hand, an asymmetric step motor is still widely utilized.The conventional self-bearing technology does not teach or suggest anycontrol algorithms for allowing such an asymmetric step motor to be usedas a self-bearing motor in the absence of the bearing coils.

In addition, the step motor causes the excited state of theelectromagnets to be changed in regular order so as to rotate the rotor.The sequential change of the driving electromagnets according to such aphase change is one of the difficulties in developing the conventionalstep motor into the self-bearing step motor. In particular, since acircumferentially overlapped length of the stator and rotor teeth ischanged upon rotation of a shaft of the motor, an overlapped sectionalarea through which the magnetic flux flows is also changed. Thus, changeof magnetic force magnitude due to the sectional area change is also oneof the causes of obstruction to the development of the self-bearing stepmotor.

DISCLOSURE OF INVENTION

The present invention is contemplated to solve the above problems.

An object of the present invention is to provide a self-bearing stepmotor system and a control method thereof, wherein the step motor itselfcan perform both motor and bearing functionalities without adding coilsfor performing the bearing functionality to a conventional step motorhaving an asymmetric arrangement of stator teeth.

According to an aspect of the present invention, there is provided aself-bearing step motor system, comprising a self-bearing step motorincluding a rotor having a plurality of rotor teeth, a stator having aplurality of stator teeth, a plurality of separate windings woundrespectively around the plurality of stator teeth, and a plurality ofsensors for measuring geometrical deviation of the rotor; and acontroller which transforms a torque current for causing the rotor torotate and control currents for performing a bearing function ofsupporting the rotor into predetermined supply currents capable ofgenerating magnetic force for allowing the rotor to be supported whilecausing the rotor to rotate in accordance with outputs of the sensorswhich are fed back to the controller and position angles of theplurality of windings, and simultaneously distributes the supplycurrents among the plurality of windings grouped so that the same phasecurrents can be supplied thereto.

Further, the controller of the self-bearing step motor of the presentinvention preferably comprise a motor controller for outputting thepredetermined torque current, a plurality of bearing controllers foroutputting the control currents based on the outputs of the plurality ofsensors fed back thereto, and a current regulator for generating thesupply currents by adding up the predetermined torque current and thecontrol currents adjusted in accordance with the position angles of therespective windings and for distributing the supply currents among theplurality of the windings grouped so that the same phase currents can besupplied thereto.

Furthermore, in the self-bearing step motor system of the presentinvention, the plurality of sensors includes a horizontal sensor formeasuring a horizontal deviation of the rotor and a vertical sensor formeasuring a vertical deviation of the rotor; the plurality of bearingcontrollers include a horizontal bearing controller for outputting ahorizontal control current based on a fed back output of the horizontalsensor and a vertical bearing controller for outputting a verticalcontrol current based on a fed back output of the vertical bearingsensor; and the current regulator distributes, among the plurality ofwindings grouped so that same phase currents can be supplied thereto,the supply currents obtained by multiplying the horizontal controlcurrent by a cosine value of the position angle of each winding,multiplying the vertical control current by a sine value of the positionangle of each winding, and adding the two multiplied values to theoutput of the motor controller.

According to another aspect of the present invention, there is provideda method for controlling a self-bearing step motor including a rotorhaving a plurality of rotor teeth, a stator having a plurality of statorteeth, a plurality of separate windings wound respectively around theplurality of stator teeth, and a plurality of sensors for measuringgeometric deviation of the rotor, comprising the steps of selecting theplurality of windings grouped so that the same currents can be suppliedthereto, transforming a torque current for causing the rotor to rotateand control currents for performing a bearing function of supporting therotor into predetermined supply currents capable of generating magneticforce for allowing the rotor to be supported while causing the rotor torotate in accordance with outputs of the sensors which are fed back tothe controller and position angles of the plurality of windings, anddistributing the transformed supply currents among the plurality ofselected windings.

In the method of the present invention, the plurality of sensors includea horizontal sensor for measuring a horizontal deviation of the rotorand a vertical sensor for measuring a vertical deviation of the rotor,and the step of transforming the torque current and the control currentsinto the supply currents is performed by multiplying the horizontalcontrol current fed back from the output of the horizontal sensor by acosine value of the position angle of each winding, multiplying thevertical control current fed back from the output of the vertical sensorby a sine value of the position angle of each winding, and adding thetwo multiplied values to the predetermined torque current.

According to the present invention, a step motor having a generalconstitution can be used as a self-bearing step motor regardless ofwhether the stator teeth 21 are arranged symmetrically andasymmetrically. Further, the self-bearing step motor can be controlledby disconnecting the connection of the windings of the conventional stepmotor, utilizing sensors for detecting geometric deviation of the rotor,and simultaneously distributing currents having the same phasescorresponding to position angles of the stator teeth among a pluralityof windings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view of a conventional self-bearing step motor.

FIG. 2 is a schematic view showing a general constitution of a stepmotor.

FIG. 3 is a schematic view of a self-bearing step motor according to anembodiment of the present invention.

FIG. 4 is a schematic view illustrating forces generated from theself-bearing step motor according to the embodiment of the presentinvention.

FIG. 5 is an enlarged view showing an overlapped portion of stator androtor teeth upon rotation of the self-bearing step motor according tothe embodiment of the present invention.

FIG. 6 is a control block diagram of the self-bearing step motoraccording to the embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, a preferred embodiment of the present invention will bedescribed in detail with reference to the accompanying drawings.

As shown in FIG. 3, a self-bearing step motor of the present inventioncomprises a rotor 10 having a plurality of rotor teeth 11; a stator 20having a plurality of stator teeth 21; a plurality of separate windings30 wound around the plurality of stator teeth of the stator,respectively; and a plurality of sensors 90, 100 for measuring geometricdeviation of the rotor 10. The plurality of windings 30 are divided intothree groups of the windings P11 to P15, P21 to P25, and P31 to P35 towhich the same phase currents are supplied, respectively. Further, apredetermined amount of current for rotating the rotor with magneticforce is simultaneously distributed to each group of the plurality ofwindings 30 supplied with the same phase current, and each group of thewindings 30 are excited. At this time, the supplied current distributedamong the plurality of windings can be changed depending on outputsignals from the plurality of sensors and positions of the plurality ofwindings.

In order to utilize a general asymmetric step motor shown in FIG. 2 as aself-bearing step motor, the respective coils or windings 30 connectedin series are disconnected from one another so that the disconnectedcoils of the stator can be separately excited, as shown in FIG. 3. Forthe sake of convenience, the respective electromagnets (coils) aredenoted by P_(jk) and classified into five segments which include setsof electromagnets supplied with different phase currents, respectively.Here, j is a subscript used for differentiating the phases of thecurrent supplied to the electromagnets and k is a subscript used fordifferentiating the segments that the electromagnets belong to.Magnitudes of the currents supplied to the respective coils varyaccording to geometric eccentricity of the center of the rotor, andaccordingly different magnetic forces are generated from the respectiveelectromagnets. Due to the difference in the magnetic forces, the rotor10 can be supported and the geometric deviation can also be compensated.

In order to explain the magnetic forces exerted on the rotor accordingto the embodiment of the present invention, first a case where the rotorof the self-bearing step motor shown in FIG. 3 is offset by x and yalong the X and Y axes will be discussed.

At this time, each air gap between the electromagnet P_(jk) and therelevant rotor tooth is computed according to the following equation(1):h _(jk) =h _(s) −x·cos θ_(jk) −y·sin θ_(jk),  (1)

-   -   where h_(s) is a radial air gap with the rotor in a centered        position of each of the electromagnets (coils), i.e. the stator        teeth, and θ_(jk) is an angle of a circumferential position of        each electromagnet measured with respect to a reference        position. The angle depends on the number of the electromagnets        N_(s) and the number of the segments N_(k), and is computed        according to the following equation:        θ_(jk)=(2π/N _(k))(k−1)+(2π/N _(s))(j−1)  (2)

By using the equations (1) and (2), the air gaps between the rotor teethand the electromagnets located at arbitrary angular positions can becomputed irrespective of whether or not the stator teeth are arrangedsymmetrically in an angular direction.

In theory, if the number of the electromagnets driven at once is threeor more, all the step motors can be utilized as the self-bearing stepmotor irrespective of whether the electromagnets are arrangedsymmetrically in the angular direction.

The differences in the air gaps of the respective driven electromagnetshave direct influences on the amount of control currents forcompensating the differences. Thus, a supply current flowing througheach electromagnet coil can be expressed as the following equation (3):i _(jk) =i _(t) +i _(x) cos θ_(jk) +i _(y)sin θ_(jk),  (3)

-   -   where i_(jk) is a j^(th) phase supply current flowing through        the electromagnet coils in the k^(th) segment, i_(t) is a torque        current, and i_(x) and i_(y) are control currents for the        bearing functionality which are added to the coils for        compensating a position of the rotor. That is, as shown in FIG.        4, if the electromagnets supplied with a 3^(rd) phase (j=3)        current are to be driven, the electromagnets (coils) P31, P32,        P33, P34, and P35 corresponding to the windings supplied with        the 3^(rd) phase current are supplied with the current and thus        excited. The air gap h_(3k) of each electromagnet supplied with        the 3^(rd) phase current can be computed from the equation (1)        by using a value of the eccentricity measured from the X and Y        directional sensors, and the current i_(3k) supplied to each        electromagnet can be computed from the equation (3). Because of        the above geometric relationships among the electromagnets        supplied with the same phase currents, a summation of the second        and third terms, i.e. Σi_(x) cos θ_(jk)+Σi_(y) sin θ_(jk), in        the equation (3) is always zero for all of the windings supplied        with the 3^(rd) phase current. That is, when the control        currents are added to an arbitrary electromagnet in order to        compensate the position of the rotor, the control currents are        correspondingly subtracted from the other electromagnets.        Accordingly, the summation of all the control currents is zero.

FIG. 5 is an enlarged view of teeth portions of the rotor and stator forillustrating an influence on bearing rigidity, which is exerted by achange in an overlapped length of the stator teeth and rotor teeth uponrotation of the rotor.

A tangential directional magnetic force Ft and a normal directionalmagnetic force Fn are determined based on geometrical and magneticcharacteristics of the electromagnets and teeth. In general, the stepmotor uses Maxwell force by which the tangential and normal directionalmagnetic forces are generated. Further, the tangential magnetic force isproportional to the square of the supply current and inverselyproportional to the air gap, whereas the normal directional magneticforce is proportional to the square of the supply current and inverselyproportional to the square of the air gap. If it is assumed that x- andy-direction displacements and the control current are extremely smallerthan the air gap and the torque current at a steady state of the motor,respectively, x- and y-direction components of the two normal andtangential directional magnetic forces can be linearized as expressed inthe following equations (4) and (5):F _(xj) =K _(q) x−K _(qc) y+K _(i) i _(x) −K _(ic) i _(y)  (4)F _(yj) =K _(qc) x+K _(q) y+K _(ic) i _(x) +K _(i) i _(y),  (5)

-   -   where K_(q) and K_(qc) are displacement stiffness of a system,        and K_(i) and K_(ic) are current stiffness for controlling the        system. In case of a variable reluctance (VR) type self-bearing        step motor having the constitution according to the embodiment        of the present invention shown in FIG. 3, the stiffness can be        computed according to the following equations (6) and (7):        K _(q)=(μ₀ LWN _(k) N ² i _(t) ²)/h _(s) ³  (6)        K _(i)=(μ₀ LWN _(k) N ² i _(t))/h _(s) ²  (7)

As expressed in the equations (6) and (7), the displacement stiffnessand the current stiffness are related to the overlapped length 8 shownin FIG. 5. As the rotor is rotated, the overlapped length is changed andthe stiffness of the system is also linearly changed. However, since thechange of the overlapped length has an influence on the currentstiffness directly affecting control signals as well as the displacementstiffness, a little mutual canceling effect can be obtained. Inaddition, due to a continuous fed back, system performance is not muchinfluenced by the change of the overlapped length. However, in a casewhere it is designed such that a minimum overlapped length is too small,particularly a case where the minimum overlapped length is zero orlower, the displacement stiffness and the current stiffness aretheoretically zero if a fringing effect is ignored. Thus, it isdifficult to perform optimal control of the system. If a permanentmagnet (PM) type step motor is to be utilized as the self-bearing stepmotor, the permanent magnet effect should be considered into theequations (6)and (7).

FIG. 6 schematically shows a block diagram of a control system forcontrolling the self-bearing step motor. In FIG. 6, there is shown acurrent regulator 120 for distributing and supplying currents to theelectromagnets in regular order so as to excite the electromagnets. Thecurrent regulator 120 is also called a current distributor. In order todrive or rotate the step motor, the excited state of the electromagnetsfor generating the magnetic forces should be changed in the regularorder. The current regulator 120 performs a function of adjusting themagnitude of the supply currents to the respective coils while allowingthe conventional step motor to operate with the same frequency as thestep motor driving frequency of a controller 110.

Hereinafter, the operation of a fed back control circuit for theself-bearing step motor will be explained in detail with reference toFIG. 6. The X- and Y-direction (horizontal and vertical direction)displacement sensors 90, 100 measure the geometric eccentric positionsx, y of the rotor. Signals corresponding to the positions measured bythe sensors pass through the X- and Y-direction transducers 91, 101,respectively. Then, the signals are compared with reference inputsignals and inputted into horizontal and vertical direction bearingcontrollers 92, 102, respectively. The horizontal and vertical directionbearing controllers generate and output horizontal and vertical controlcurrents i_(x), i_(y). Further, a motor controller 110 generates andoutputs a predetermined torque current i_(t) for causing the rotor torotate. The current regulator 120 receives the torque current i_(t) andthe horizontal and vertical control currents i_(x), i_(y), transformsthem into the supply currents i_(k) to be supplied to the respectivewindings, and outputs the supply currents at once. The supply currentssupplied simultaneously to the respective windings are distributed amongand supplied to the respective electromagnets, through the followingprocedures. That is, magnitudes of the horizontal and vertical controlcurrents are first adjusted in accordance with the second and thirdterms of the equation (3) depending on the angular positions of thewindings to be supplied with the currents. Then, the control currentsi_(x), i_(y) are added to the torque current i_(t), and thus, the supplycurrents are obtained. At this time, the adjusted part of the controlcurrents in each of the supply currents (i.e., the currents obtainedfrom the second and third terms of the equation (3)) can be added to orsubtracted from the torque current in accordance with the geometricdeviation of the rotor, but the summation of the adjusted part of thecontrol currents is always zero.

Thus, by utilizing the self-bearing step motor and controller shown inFIG. 6, the self-bearing step motor can be efficiently controlledwithout provisions of the mechanical bearings and additional bearingcoils and irrespective of symmetric or asymmetrical arrangement of thestep motor.

INDUSTRIAL APPLICABILITY

According to the present invention constructed as such, the self-bearingstep motor of the present invention can replace the conventional stepmotor. In particular, owing to advantages of the magnetic levitationtechnology, the self-bearing step motor can be used semipermanently evenin severe environments in which high cleanness and vacuum are required,without a need for maintenance thereof. Thus, wide industrialapplication thereof can be obtained. Further, the self-bearing stepmotor of the present invention can be applied to computer disk drivespindles, bio-pumps, canned motor pumps, etc. Furthermore, theself-bearing step motor can also be manufactured as a micro actuator.

It should be understood that the embodiment of the present inventiondescribed above and illustrated in the drawings is not construed aslimiting the technical spirit of the present invention. The scope of theinvention is defined only by the appended claims, and those skilled inthe art can make various changes and modifications to the embodiment ofthe present invention within the scope of the invention. Thus, suchchanges and modifications fall within the scope of the invention as faras they are obvious to those skilled in the art.

1. A self-bearing step motor system, comprising: a self-bearing stepmotor including a rotor having a plurality of rotor teeth, a statorhaving a plurality of stator teeth, a plurality of separate windingswound respectively around the plurality of stator teeth, and a pluralityof sensors for measuring geometrical deviation of the rotor, theplurality of sensors includes a horizontal sensor for measuring ahorizontal deviation of the rotor and a vertical sensor for measuring avertical deviation of the rotor; and a controller which transforms atorque current for causing the rotor to rotate and control currents forperforming a bearing function of supporting the rotor into predeterminedsupply currents capable of generating magnetic force for allowing therotor to be supported while causing the rotor to rotate in accordancewith outputs of the sensors which are fed back to the controller andposition angles of the plurality of windings, and simultaneouslydistributes the supply currents among the plurality of windings groupedso that same phase currents can be supplied thereto, the controllercomprising: a motor controller for outputting the predetermined torquecurrent: a plurality of bearing controllers for outputting the controlcurrents based on the outputs of the plurality of sensors fed backthereto, the plurality of bearing controllers include a horizontalbearing controller for outputting a horizontal control current based onan output of the horizontal sensor which is fed back to the horizontalbearing controller and a vertical bearing controller for outputting avertical control current based on an output of the vertical bearingsensor which is fed back to the vertical bearing controller; and acurrent regulator for generating the supply currents by adding up thepredetermined torque current and the control currents adjusted inaccordance with the position angles of the respective windings and fordistributing the supply currents among the plurality of the windingsgrouped so that the same phase currents can be supplied thereto, thesupply currents obtained by multiplying the horizontal control currentby a cosine value of the position angle of each winding, multiplying thevertical control current by a sine value of the position angle of eachwinding, and adding the two multiplied values to the output of the motorcontroller.
 2. The self-bearing step motor system as claimed in claim 1,wherein the plurality of stator teeth and windings are arranged togenerate magnetic flux in a rotor axial direction.
 3. A method forcontrolling a self-bearing step motor including a rotor having aplurality of rotor teeth, a stator having a plurality of stator teeth, aplurality of separate windings wound respectively around the pluralityof stator teeth, and a plurality of sensors for measuring geometricdeviation of the rotor, the plurality of sensors include a horizontalsensor for measuring a horizontal deviation of the rotor and a verticalsensor for measuring a vertical deviation of the rotor, the methodcomprising the steps of: selecting the plurality of windings grouped sothat the same currents can be supplied thereto; transforming a torquecurrent for causing the rotor to rotate and control currents forperforming a bearing function of supporting the rotor into predeterminedsupply currents capable of generating magnetic force for allowing therotor to be supported while causing the rotor to rotate in accordancewith outputs of the sensors which are fed back to the controller andposition angles of the plurality of windings, the transforming thetorque current and the control currents into the supply currents isperformed by multiplying a horizontal control current fed back from theoutput of the horizontal sensor by a cosine value of the position angleof each winding, multiplying the vertical control current fed back fromthe output of the vertical sensor by a sine value of the position angleof each winding, and adding the two multiplied values to thepredetermined torque current; and distributing the transformed supplycurrents among the plurality of selected windings.
 4. The method asclaimed in claim 3, wherein the plurality of stator teeth and windingsare arranged to generate magnetic flux in a rotor axial direction.